1. Field of the Invention
The present invention relates to a conductive paste, a solar cell in which a grid electrode made of the conductive paste is formed, and a fabrication method for the same.
2. Description of the Background Art
A solar cell has a p-type silicon (Si) semiconductor substrate, an n-type layer formed in the surface region of the semiconductor substrate, and a conductive electrode connected to the n-type, layer. More specifically, as schematically shown in FIG. 1 and FIG. 2, a p-type silicon semiconductor substrate has typically a thickness of about 500 .mu.m, and an n-type layer 2 is formed in the surface region of the p-type Si semiconductor substrate 1 to a depth on the order of 0.3 .mu.m to 0.5 .mu.m. Also, on the surface of the p-type Si semiconductor substrate 1 on which the n-type layer (the front surface) 2 is formed, grid electrodes 3 for extracting the negative (minus) potential from the n-type layer 2 and an antireflection coating 4 for increasing cell efficiency by means of a light confinement effect are formed. Also, on another surface of the p-type Si semiconductor substrate 1, the back surface electrode 5 for extracting the positive (plus) potential is formed. The busbars 6 for external connection are formed on the n-type layer 2 so as to be connected with all of the grid electrodes 3.
Such a solar cell is generally fabricated according to the following sequence. That is, firstly, the n-type layer 2 is formed on a surface of the p-type Si semiconductor substrate 1 by diffusion of n-type dopant impurities. Then an antireflection coating 4 comprising such oxides as SiO.sub.2, TiO.sub.2 and the like is formed on the surface of the n-type layer 2. Thereafter, an already partially prepared conductive paste, e.g. silver (Ag) paste, is screen printed and baked to form grid electrodes 3.
Then, by screen printing the conductive paste on another surface of the p-type Si semiconductor substrate 1 and baking it, a back surface electrode 5 is formed over the entire surface or in a spider web shape. Note that there are often cases where the back surface electrode 5 has a double layer structure of an aluminum (Al) electrode layer and a silver electrode layer. Subsequently, after pre-soldering is performed on the p-type Si semiconductor substrate 1 on which the grid electrodes 3 and the back surface electrode 5 are formed, terminal portions 6 for external connection are formed on the grid electrodes 3 by solder reflow, whereby the solar cells are completed.
However, in the case where the grid electrodes 3 are formed on the antireflection coating 4 in the conventional Solar cells, the insulative property of the antireflection coating 4 intervenes and satisfactory ohmic contact between the n-type impurity layer 2 and the grid electrodes 3 cannot be attained. As a result, when forming the grid electrodes 3, the antireflection coating 4 must be masked in anticipation of the formation positions of the grid electrodes 3 and pattern etching of the antireflection coating 4 must be performed, and it is further necessary to accurately perform position matching when printing the conductive paste. The requirements for such extremely complicated processes causes the disadvantage of incurring a large scale increase in the manufacturing cost.